Aircraft structures have traditionally been manufactured primarily of such metals as aluminum, titanium and steel alloys. However, the current trend is to manufacture many previously metal structures from composite materials such as carbon fibers impregnated with an appropriate resin. The particular resin chosen causes the composite material to be either thermoplastic or thermosetting. Thermoplastic composite materials become soft and pliable when heated so that they may be repeatedly reformed by heating. Thermosetting composite materials are normally pliable before they have been heated. However, they are cured by the application of heat. Thus, once thermosetting composite materials have been exposed to heat, they permanently lose their pliability.
Composite materials used to fabricate aircraft structures--whether they be thermoplastic or thermosetting--are normally available in relatively thin sheets or tapes having their fibers extending in a single direction. These unidirectional sheets or tapes are normally stacked several layers deep in order to increase the thickness and hence strength of the resulting composite structure. Also, the angular orientation of the fibers in each layer can be selected to tailor the strength characteristics of the structure in each direction. For example, by orienting the fibers in most of the layers in a longitudinal direction, a composite structure can be made stronger longitudinally than it is transversely.
As mentioned above, the process of laminating sheets of composite materials begins with the stacking of the composite sheets to form a laminate. The sheets are stacked on a tool or mold having a surface that conforms to the desired shape of the composite structure. When the sheets are initially stacked, the laminate is substantially thicker than the finished product because of the presence of air gaps in the laminate resulting from waviness in the composite sheets. The laminate is compressed to the thickness of the finished product and formed into an integral mass through the combined application of heat and pressure. If the tool on which the laminate is placed is flat, the pressure may sometimes be applied with a conventional press. However, it is often necessary for the composite structure to have a complex curvature. Under these circumstances, it would be necessary to machine the surface of the tool to this complex curvature, to machine the contour of the pressure plate of the press to exactly match the complex curvature of the tool, and to then position the pressure plate and tool in exact registration with each other. It is very difficult and hence expensive to produce a press that can meet these requirements, especially where the size of the composite structure is large, thus requiring a large press. Conventional presses may also be inadequate even if the surface of the tool is completely flat. It is important that the pressure on the laminate be relatively constant. However, variations in the thickness of individual sheets in the laminate can make the laminate thicker in some areas. As a result, the pressure that the press applies to the laminate will be greater in these areas. Similarly, the pressure on the laminate will also be less in areas where localized variations in the thickness of the sheets make the laminate relatively thin in some areas. Thus conventional presses may be inadequate even where the tool is flat and the pressure plate of the press is also perfectly flat and positioned parallel to the tool during the laminating process.
The above-described limitations of conventional presses for use in the laminating process have led to the widespread use of autoclaves to laminate composite sheets. In autoclave processing of composite sheets, a predetermined number of sheets are stacked on a tool having the desired shape of the composite structure. A porous breather cloth, such as a Fiberglas.RTM. cloth, is placed over the laminate. The breather cloth is then covered by an air-impermeable sheet, and the edges of the air-impermeable sheet are sealed to the surface of the tool so that the air-impermeable sheet, in combination with the tool, forms a vacuum bag. Air is generally evacuated from a vacuum port in the tool so that the pressure differential between the interior and exterior of the bag is reflected as a pressure exerted on the laminate. The laminate and vacuum bag assembly is then placed in an autoclave. The autoclave is basically a closed container that is heated and pressurized. The pressure differential between the evacuated interior of the vacuum bag and the pressurized autoclave consolidates the laminate, while the heat causes the composite sheets in the laminate to adhere to each other to form a unitary structure. It is important to recognize that autoclave processing of composite laminates avoids the two above-described limitations of conventional presses. First, the pressure applied to the laminate will be constant regardless of the surface contour of the tool. Thus composite structures having complex curvatures may be manufactured easily. Second, the pressure applied to the laminate will be constant regardless of variations in the thickness of individual sheets in the laminate.
Although there are many advantages to using autoclaves to laminate composite structures, there are also some problems. When the vacuum bag is evacuated during the autoclaving process, the vacuum can draw resin from out of the laminate at its edges. Two approaches have been developed to deal with this problem. First, a nonstructural, resin-impermeable dam has been placed around the laminate in contact with its edges. Such dams have generally been fabricated from cork or wood. The dam, by contacting the edges of the laminate, prevents the flow of resin from the edges of the laminate. The second solution to the resin bleed problem has been to delay the evacuation of the vacuum bag for a predetermined period after heat has been applied to the laminate. Heat causes the viscosity of the resin in the laminate to increase. After the viscosity has increased sufficiently, the resin will not flow from the laminate when the vacuum is applied.
A second problem encountered in processing composite laminates with an autoclave is wrinkling or buckling of the laminate. This phenomena appears to be caused by transverse pressure that the vacuum bag exerts on the edges of the laminate during the consolidation process when the thickness of the laminate is being reduced from its unconsolidated thickness to the thickness of the final structure. The problem does not occur with composite materials using thermosetting resins. It is believed that the heat applied to the laminate prior to consolidation causes curing of thermosetting composite materials so that they are not sufficiently pliable to wrinkle or buckle during consolidation. Thus the problem remains unsolved only with autoclave processing of thermoplastic composite materials. Furthermore, the problem appears to be more serious with the higher autoclave pressures used to process relatively large and/or complex thermoplastic composite structures.